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The determination of transect size is of great importance in the projects where Remote Sensing techniques are used. In these projects the covered areas are generally widespread and the cost of the fieldwork must be balanced with the total cost of the project. The optimal transect size to supervise remote sensing techniques is determined in the present work. For this, the complex relationship between the vegetation of semi-arid with reflectance and space resolution of TM/Landsat images was considered . The information of six field campaigns in a large region of the Brazilian northeast was used. Once the type of vegetal cover which better explains the values of reflectance of the images was determined, the optimal transect size of the fieldwork was specified. It allows to obtain reliable data to supervise remote sensing techniques. At a first stage, it was determined that the total and arboreal vegetal cover explains better the values of reflectance in the visible spectral images. Although the correlations are not high, they are consistent for change detection in the visible range. Using the Pearson-Hartley graphics of Power of F Test, it was specified that 45 m would be the minimum length of transect. This size would allow to obtain excellent data from the vegetation of Caatingas abertas (shrub steppes) up to Caatingas arboreas (dry forest). Based on this and the size of pixels of the images, the sampling with transects of 50 m with north-south direction was recommended. This size and direction will allow to obtain data of the land with 80% of probability of representing a TM/Landsat pixel value. [ABSTRACT FROM AUTHOR]